In conventional orthodontic treatment, orthodontic brackets are mounted to the surfaces of a patient's teeth, and an arch-shaped orthodontic wire is fastened to the brackets with ligature wire. The arch-shaped wire imparts forces to the teeth, which are created by the bending and resultant tension within the wire. The wire is shaped so that the forces exerted by the wire move the teeth, so as to correct the malocclusion of the patient's dental arch.
Recently, alloy wires exhibiting shape memory properties, such as nickel-titanium (Ni-Ti) alloy wires, have been used to make orthodontic wires. Alloy wires possessing shape memory properties are readily amenable to change in shape at low temperatures, but can be reformed to their original configurations when heated to suitable transition temperatures. The alloy wires exhibiting shape memory properties, such as the Ni-Ti alloy wires, are frequently referred to as shape memory alloy wires Some shape memory alloy wires, like the Ni-Ti alloy wires, exhibit excellent superelastic and springback properties. Superelasticity occurs when the stress value remains substantially constant up to a certain point of wire deformation, and when the wire deformation rebounds, the stress value again remains substantially constant. Therefore, when a Ni-Ti archwire, for example, is subjected to a load to create a deflection, the load remains substantially constant throughout a given superelastic zone of deflection of the wire. Moreover, because such shape memory wires possess excellent springback properties, they can be deflected to greater degrees than other types of wires, without causing permanent deformation of the wire.
Orthodontic shape memory alloy wires are formed so as to retain particular shapes. The shape of an orthodontic wire is determined depending upon the malocclusion of a patient's teeth, in order to exert forces and thus move the teeth to correct the malocclusion. When a shape memory alloy wire is mounted to the orthodontic brackets on a patient's teeth, the wire is deflected and tends to springback to the particular shape previously imparted to the wire, thus applying a force and in turn shifting the teeth. While such use has occurred, it has failed to provide a method or apparatus for shaping wires made of shape memory alloys so as to retain a particular shape in a manner that is simple, accurate, and relatively inexpensive to perform.
One method of imparting shapes to shape memory alloy wires is shown in Japanese patent Early Disclosure 58-50950 (1983), wherein restraining channels are formed into a mold or pattern made of gypsum or glass. Each restraining channel is made in the shape that a wire should take in order to correct a particular patient's malocclusion. The wire is then fitted into the restraining channel and heated to a temperature sufficient to cause the wire to retain the shape of the channel. Therefore, when the shape memory wire is deflected within its elastic range, it will springback into the particular shape imparted by the channel.
One problem with the gypsum or glass molds is that they are relatively time consuming and therefore expensive to build. Each mold or pattern must be formed so that its restraining channel imparts the unique shape to the orthodontic wire required to meet the needs of an individual patient. Because one patient's malocclusion is usually different than another's, it is usually necessary to prepare an individual mold or pattern for each patient. Therefore, the gypsum or glass molds are generally not economically feasible because each mold usually can only be used for one patient. Moreover, it is frequently necessary to impart three-dimensional shapes or, that is, shapes formed in more than one plane to orthodontic wires. With gypsum or glass molds, however, it is usually difficult to form the molds to impart the three-dimensional shapes, thus making the use of molds even more time consuming and expensive.
Under another known method of imparting shapes to shape memory alloy wires, each end of a shape memory alloy wire is gripped with a pair of electric pliers. The pliers are typically coupled to a constant-current generating apparatus. Electrode plates mounted within the pliers pass electric current through the wire in order to heat the wire. While heating the wire, the operator bends the wire into a desired shape by manipulating the pliers. The operator then tries to hold the wire in place until the wire reaches a temperature sufficient to cause the wire to retain the new shape. One problem with this method is that it is usually difficult for the operator to maintain the wire in the same shape throughout the heating process. As a result, the exact shape required is frequently not accurately imparted to the shape memory alloy wire.
Therefore, it is an object of the present invention to provide a method and apparatus for imparting shape to shape memory alloy wires that overcome the problems of known methods and apparatus.